TYPO3 Core

As stated in https://support.google.com/webmasters/answer/183668?hl=en#general-guidelines the URLs in sitemap should be "fully-qualified URLs. Google will crawl your URLs exactly as listed. For instance, if your site is at https://www.example.com/, don't specify a URL as https://example.com/ (missing www) or ./mypage.html (a relative URL)."

Our config:
No domain in sys_domain or TypoScript.

Site-Configuration:

site:
  rootPageId: 1
  base: /
  languages:
    -
      languageId: '0'
      title: Deutsch
      navigationTitle: Deutsch
      base: /
      locale: de_DE.UTF-8
      iso-639-1: de
      hreflang: de-DE
      direction: ltr
      typo3Language: de
      flag: de
      enabled: true
  errorHandling: {  }

Non-composer
TYPO3 9.4.0
PHP 7.2